A conventional method for measuring air flow in the air conditioning industry is by sensing the air velocity in a duct or plenum with a velocity pressure sensing device. This method, however, is not accurate or reliable at low air flow levels due to the present state of the art of instrumentation and the nature of velocity pressure vs. air flow relationship. When reliable, accurate measurement of air flow less than 300 fpm is required, either high cost instrumentation and/or devices that impose unnecessary pressure loss and excessive noise, must be employed to attain accurate sensing of such velocities. The applicability and accuracy of these devices is also limited by the wide range of flow conditions that they must typically handle for Variable-Air-Volume systems. Variable cross sectional area devices are known which require a manual adjustment of the open area or the insertion of an aperture plate to achieve a given velocity/flow relationship.
It is, therefore, an object of the present invention to provide an air flow measuring device which will measure more accurately greater ranges of air flow in air ducts and plenums.
Another object of the present invention is to provide a cheaper and more simple device for measurement than known expensive instrumentation.
A still further object of the present invention is to provide a device which is less noisy and minimizes pressure loss at a maximum air flow without the sacrifice of reduced cross sectional area at low air flows or the need to readjust or remove a cross-sectional area reduction element.
Another object of the present invention is to provide an air flow measuring device which does not require a manual adjustment of the open area to achieve a given velocity/flow relationship.
A still further object of the present invention is to provide an air flow measuring device where the cross-sectional area of the air flow is automatically varied by the velocity pressure exerted against the rotatable damper blades.
Another object of the present invention is to provide a device which will permit continuous readings to be made throughout the entire range of air velocity flow without the need to manually adjust the opening or insert an aperture plate.